1. The present invention relates generally to spacecraft and, more particularly, to spacecraft solar arrays.
2. Description of the Related Art
Spacecraft typically carry solar cells as a primary energy source with rechargable batteries providing energy storage for eclipse operations. The solar cells are positioned and oriented on the spacecraft so that they are exposed to solar radiation.
On spinning spacecraft, solar cells are generally arranged about the outside of a spinning spacecraft body. Accordingly, only a fraction of the cells are exposed to solar radiation at any instant in time. On body-stabilized spacecraft, in contrast, solar cells are typically arranged in planar arrays and carried on solar wings which extend from opposite sides of a spacecraft body. Preferably, the solar wings rotate to keep them as orthogonal to the solar radiation as possible. Because the solar wings can be quite long in their deployed configuration, they are generally formed of a plurality of planar solar panels which are coupled together in an accordion arrangement so that they can be collapsed to a smaller stowed configuration for spacecraft launch.
The number of solar cells that must be carried by a spacecraft is a function of the anticipated spacecraft power demand and the efficiency of the solar cells. Although high-efficiency solar cells reduce the number of cells required by a specific spacecraft, they are quite expensive. Because weight and weight-related costs also increase with the number of solar cells, there is a considerable incentive to reduce the quantity of solar cells that a spacecraft must carry.
Accordingly, efforts have been extended to concentrate solar radiation upon solar cells by using reflective surfaces that are positioned adjacent to solar panels and oriented to reflect additional radiation onto the cells. Solar radiation that would otherwise have passed by a solar wing is thus redirected to be incident upon the solar cells. Although a solar cell's efficiency in conversion of this additional reflected radiation to useful energy is typically less than it is for the directly incident radiation, primarily due to increased cell temperature and decreased angle of incidence, solar concentration allows the number of spacecraft solar cells to be significantly reduced with consequent savings in spacecraft weight and cost.
Both rigid and flexible reflectors have been proposed for solar radiation concentration with flexible reflectors generally having a weight advantage. An exemplary flexible reflector system is shown in U.S. Pat. No. 4,282,394. In this system, reflector arms are carried on both inboard and outboard frames. Each of the reflector arms is formed of a plurality of hinged arm sections and each arm section of the inboard frame carries a reflective plastic sheet that is wound on a spring-biased roll. An end of each sheet is attached to a respective arm section on the outboard frame.
During deployment, an extensible shaft moves the outboard frame away from the inboard frame and each reflective sheet is unrolled to reflect solar radiation onto solar cells. Although this reflector system concentrates solar radiation, its complex structure (e.g., hinged arms, inboard and outboard frames and extensible shaft) significantly contributes to spacecraft weight and cost.
In an exemplary Naval Research Laboratory design, a single thin-film reflector spans a plurality of solar panels that are coupled together in an accordion arrangement. Each thin-film reflector is carried with tension springs between a pair of rotatable booms. Because the edges are configured to assume a catenary shape, the reflector film is held very flat under near-uniform inplane tension. In order to fold the solar panels into a stowed position, the booms rotate to lie alongside the panels and the thin-film reflector is rolled (e.g., from the reflector center) so that it lies parallel to the booms. Although this reflector system is potentially lighter and simpler than the system described above, it still involves numerous mechanical parts (e.g., booms, cables and pulleys) which have significant weight and complexity.
Other reflector systems are described in U.S. patent application Ser. No. 08/081,909, filed Jun. 18, 1993 and now abandoned (as a continuation of application Ser. No. 07/802,972, filed Dec. 6, 1991 and now abandoned), titled "Augmented Solar Array with Dual Purpose Reflectors" and assigned to Hughes Electronics, the assignee of the present invention. In an exemplary system, a reflector is formed from a reflective material (e.g., an aluminized polyimide film) that is carried by a peripheral frame or affixed over a ribbed structure or a thin metal sheet. Each reflector is coupled to a solar panel by a hinge mechanism. Prior to spacecraft launch, the reflector is rotated to lie proximate to the solar cell face of the solar panel. After launch, the hinge mechanism rotates the reflector to a position in which it forms a deployment angle with the solar cell face. In an exemplary hinge mechanism, a hinge spring urges the reflector to rotate away from the solar cell face. The hinge mechanism includes a stop member which halts this rotation when the reflector reaches the deployment angle.
In another reflector system embodiment, reflectors are fabricated by suspending a reflective film between a pair of flexible rods that are rigidly coupled to a solar panel. The rods are typically tethered such that the reflectors lie parallel to the solar cell face prior to spacecraft launch. Deployment is effected by untethering which allows the rods to whip directly to a position in which the reflective film forms a deployed angle with the panel.
Although the latter reflector system effectively redirects radiation, the solar reflectors are stowed over the solar cell face of the solar panels. Accordingly, they block the use of the solar panels during any period (e.g., a transfer orbit) in which the solar panels are in a storage position that prevents reflector deployment.